1. Field of the Invention
The present invention relates generally to methods and systems for reducing electromagnetic interference (EMI) in electronic circuits. More particularly, the present invention relates to methods and systems for generating spread spectrum clocks to reduce EMI in electronic circuits.
2. Background Art
EMI is a radiated or conducted energy that adversely affects circuit performance. Many types of electronic circuits radiate or are susceptible to EMI and must be shielded to ensure proper performance. Establishing basic electromagnetic compatibility in any electronic device generally requires two distinct approaches. The first approach is to reduce EMI generated from internal sources. This may be best accomplished by designing an electronic circuit or device so that it inherently generates less EMI. Residual EMI may then be suppressed or contained within the enclosure by appropriate filtering and shielding methods. Filtering cables at the point where they enter or leave the enclosure will reduce conducted emissions. Radiated EMI may be eliminated or reduced by the use of shielded enclosures and shielding materials. The second method for establishing electromagnetic compatibility (EMC) in a device is to improve its immunity (or reduce its susceptibility) to interference from external EMI sources. Susceptibility to external EMI may be reduced or even eliminated by designing circuits and choosing components which are inherently less sensitive to interference. As in the case of internal sources, conducted EMI may be reduced with filtering devices on incoming and outgoing leads, and susceptibility to externally radiated EMI may be reduced with use of effective shielding.
Government regulations in the US and many other countries prohibit electronic products from emitting EMI that could interfere with radio and television receivers. European regulations also include EMI immunity levels. Manufacturers of commercial electronic products generally contend with three types of EMI problems. The first problem is suppression of internally generated signals to prevent excessive levels of radiated and/or conducted emissions. The FCC in the United States, CSA in Canada, VCCI in Japan, AUSTEL in Australia, and legislation by EU (European Union) member countries all set certain standards for EMI emission levels that commercial electronic devices must meet before being sold in those countries. Many electronic products sold in the US must be tested and verified or certified for compliance with the FCC's Part 15 regulations. The second problem is the external ambient interference with equipment operation, where many companies establish their own specifications for immunity to EMI over a range of phenomena. These may include electrostatic discharge (ESD), radiated immunity, and electricfast transients (EFT). This is not yet a requirement in the US; however, EU regulations currently do include immunity requirements. The third problem is the internally generated emissions interfering with equipment operation, where EMI from one circuit can interfere with the function of another within the same system or subsystem. This is typically called cross-talk, and is the most common source of system susceptibility.
It is well known that EMI increases as a result of higher clock speeds. This radiation, which is mainly produced by fundamental and low-order harmonics, unfortunately coincides and interferes with radio FM bands. This is why regulatory agencies have placed limits on electromagnetic radiation produced by any electronic instrument that might use clocks and generate emissions. Almost any electrical transitions with sharp edges, such as clocks, data, address and control, produce electromagnetic radiation. As performance requirements increase, clock speeds have also increased. The transition edge, or in engineering terms, the slew rate, has become faster and faster as the need for meeting “set up” and “hold time” has become harder to meet, where “set up” is the time needed for a data pulse to be stable before the rising edge of the clock, and “hold time” is the time for the data pulse to remain stable after the edge of the clock.
Today, clocks are no longer fed to only one or two devices on circuit boards. Rather, clocks are being distributed all over the circuit board. Further, there has been an increase in memory requirements, and other loads on the clock lines. Since EMI is linearly proportional to current, the area of the current loop, and with the square of frequency, recent demand for higher speeds in electronic devices have significantly contributed to the difficulty of meeting the requirements for reducing electromagnetic radiation in electronic devices.
One conventional scheme, which is disclosed in U.S. Pat. No. 5,488,627 (the “'627 patent”), describes a well-known phase locked loop (PLL) frequency synthesizer, with an added modulation section that includes a spread spectrum modulation generator, where the modulation is fed into a voltage controlled oscillator (VCO) to provide a desired modulation index. The modulation reduces the spectral magnitude of the EMI components at harmonics of the clock when compared to the spectrum of the same clock signal without modulation. As explained therein, although the spectral width of the spread spectrum clock signal at a harmonic is greater than the width of the non-modulated clock signal, the maximum amplitude for harmonic is reduced. In order to minimize the amplitude of the signal for all frequencies, the '627 patent teaches a symmetrical signal modulating profile in four equal quadrants, where the profiles are expressed as a percentage of frequency deviation versus a percentage of the period of the periodic wave, i.e. the first quadrant is between −25% to 0% of the period, the second quadrant is between 0% to 25% of the period, the third quadrant is between 25% to 50% of the period, and the fourth quadrant is between 50% to 75% of the period.
However, the modulating profiles in the '627 patent brings about a major drawback. As illustrated in FIG. 4, the modulating profiles in the '627 patent create some undesirable spectral anomalies, which are shown as valleys 411 and peaks 412 in FIG. 4. These spectral anomalies can adversely affect EMI reduction and the clock output signal.
Accordingly, there is a need in the art for improved and more efficient methods and systems to reduce EMI in electronic circuits.